This invention relates to scanning interferometers, and more particularly to a method and apparatus for accurately controlling the excursion of the cyclicly moving element in a scanning interferometer, such as the moving mirror in a Michelson interefometer.
Scanning interferometers are well known, and are employed, for example, in spectrometry and metrology. Thus, in multiplex spectrometry, a plurality of spectral intensities--each corresponding to a resolvable spectral interval--are simultaneously observed by a single detector, each spectral wavelength or frequency being differently encoded so that the spectrum may be transmitted over a single information channel and subsequently retrieved through a decoding process. A particularly elegant method for accomplishing the encoding makes use of a Michelson interferometer with a cyclicly moving mirror. Electromagnetic radiation passing through the interferometer is divided by a beam splitter so as to follow a pair of paths. One path is reflected by a fixed mirror, and the other, from the moving mirror. The two paths are then recombined at the beam splitter. As a result of interference, each wavelength of the recombined radiation is modulated at a different frequency, the frequency depending upon the wavelength and the velocity of the moving mirror. Individual wavelengths can be discriminated from one another by frequency filtering, the amplitude of each frequency being proportional to the intensity of the radiation at the optical wavelength or frequency corresponding to the modulation frequency. Commonly, the spectrum is extracted from the observed interferogram (i.e., the radiation intensity as a function of optical wavelength or frequency is deduced from the observed signal intensity as a function of mirror position) by numerically performing a Fourier transform.
While somewhat more sophisticated than conventional (i.e., dispersive) scanning spectrometry, multiplex spectrometry enjoys a number of advantages. For instance, as is well known, a multiplex spectrometer in effect observes each resolution element for half of the total observing time, as compared to the fraction 1/n (where n is the number of resolution elements) of the total time, as is the case for a conventional scanning spectrometer. In the case of a system which is detector noise limited (as is common in infrared spectrometry), this n/2 advantage in signal leads to a signal-to-noise advantage of (n/2).sup.1/2, an improvement known as the multiplex or the Fellgett advantage. Additionally, the Michelson interferometer enjoys an optical throughput (i.e., a field of view-aperture area product) advantage compared to slit spectrometers.
The resoluation obtained by a scanning Michelson interferometer is dependent upon the total excursion of the moving mirror, a larger resolution requiring a larger excursion. The accuracy of the representation of the spectrum depends upon the accuracy of the determination of the interferogram. This in turn requires an accurate determination of the varying retardation (i.e., the changing difference in path length travelled by the two portions of the split beam caused by the motion of the moving mirror). The interferogram ideally is an accurate replica, at a much lower range of frequencies, of the intensity pattern of the train of wave fronts incident upon the aperture of the interferometer. This ideal can be realized by an interferometer in which the moving mirror is translated at a uniform velocity for an infinite distance. In practice, the velocity need not be constant, provided that it is reasonably well-behaved and known. Nor need the retardation become infinite, provided that its limits, relative to zero retardation (i.e., relative to the condition of equal path lengths for the two beams) are also known. Indeed, it is common practice in interferometric spectrometry to limit the observations to values of the retardation between some positive and negative maximum retardations (corresponding to the desired resolution) not necessarily symmetrically located about zero retardation. Implicit in such a practice is the realization that an ideal interferogram of a constant source is symmetric about zero retardation.
As is well known in the art, the velocity or incremental displacement of the moving mirror may be readily sensed by simultaneously observing, along with the spectral source of interest, a monochromatic reference source, such as a laser. For the ideal interferometer with a constant velocity mirror, monochromatic radiation exhibits a sinusoidal interferogram, the period of the sinusoid corresponding to a change in retardation equal to the wavelength of the monochromatic source. In the case of a Michelson interferometer, this corresponds to a displacement of the moving mirror equal to one half a wavelength of the monochromatic radiation. For a non-uniformly moving mirror, corresponding signal intensities of successive cycles of the reference interferogram produced by the modulation of a monochromatic source still mark integral wavelength retardation differences, provided, of course, that the mirror velocity does not change sign in the interim. The reference interferogram may be obtained either through a reference interferometer mechanically coupled to the principal interferometer, or directly through the principal interferometer, the latter approach using a monochromatic source and associated detector chosen so as to be outside the wavelength region of the spectrum of interest.
Additionally, zero retardation must also be measured. Prior art rapid scanning interferometers require a determination of zero retardation for each scan, since the deceleration and reversal of the motion of the moving mirror occupies a time period considerably greater than that required to move the mirror through one or more fringes (i.e., cycles) of the reference interferogram. For instance, mirror velocities between 1 and 50 mm/sec are commonly employed, and typically a few hundred fringes are passed in decelerating the mirror. Consequently, the value of the retardation at reversal is not known within a wavelength of the reference radiation, and the fringe count of the reference interferogram is lost, it being impossible to determine if successive fringes arise from motion of the mirror in the same or in opposite directions. In prior art interferometers, zero retardation is commonly sensed by observing a broad-band source through an auxiliary interferometer mechanically coupled to the principal interferometer. At zero retardation, all wavelengths constructively interfere, so that a spectrally broad-band source will exhibit an interferogram with a maximum peak (a so called "white light burst"). It will be appreciated that the requirement for a separate interferometer mechanically coupled to the principal interferometer complicates and adds to the expense of a scanning interferometer.
An alternative approach is in the use of a pair of monochromatic signals of the same wavelength and substantially in phase quadrature with one another. Separate detection of each signal allows scan reversal to be unambiguously detected, and the velocity and direction of scan to be measured throughout the scan. Such dual phase reference beam systems require only an initial determination of zero retardation when the system is initialized. While this approach may be realized using only one interferometer, it will be appreciated that dual phase monochromatic sources also represent a complication and a cost burden to scanning interferometer systems.
Accordingly, it is an object of the present invention to provide a scanning interferometer that does not require an auxiliary interferometer to sense the zero retardation condition of the moving element or a dual phase reference source to unambiguously sense velocity reversal. Additionally, it is an object of the present invention to provide a scanning interferometer in which the retardation is continuously monitored.